(1) Field of the Invention
The invention relates to a device and to a method to melt glass.
(2) Description of Related Art
A device known from PCT/US9607052 comprises a melting unit in the form of a vertically positioned tube that is provided with a gas-tight and fireproof jacket. The material of which the jacket of the tube is made—normally ceramic material—is a function of the raw material to be melted and it is selected in such a way that reactions between the jacket material and the raw material to be melted are kept to a minimum. The upper end of the tube has an inlet opening through which the raw material is fed. An outlet opening that serves to discharge the melt is located in the lower section.
The prior-art melting unit is concentrically accommodated in an insulated steel casing. The annular space formed between the insulation of the casing and the ceramic tube constitutes the combustion chamber in which the heat needed for the melting process is generated by burning a gas, preferably natural gas. Thus, the material to be melted is fired indirectly. The exhaust gases that are formed during the combustion process are carried off via an exhaust gas line that exits the combustion chamber, so that the gases do not come into contact with the melt or with the raw material. Consequently, the melt removed from the melting unit contains a considerably lower fraction of inorganic impurities than the melt of conventional tank melting processes.
However, when waste glass—of the type found, for example, in mineral fiber production—that is strongly laden with organic impurities is melted, the organic constituents present in the melt give rise to so-called black hearts which can no longer be eliminated, even in a subsequent refining process.